Curable compositions containing chloroprene rubber

ABSTRACT

A vulcanizable composition comprising a chloroprene rubber and at least one amino acid selected from the group consisting of arginine, lysine, hydroxylysine, ornithine, cystine, asparagine, glutamine and citrulline, and further optionally, at least one metal oxide selected from the group consisting of magnesium oxide and zinc oxide. This invention provides also a vulcanizable composition comprising a chloroprene rubber, at least one amino acid selected from the group consisting of tyrosine, threonine, tryptophan, proline and hydroxyproline, and at least one metal oxide selected from the group consisting of magnesium oxide and zinc oxide.

This invention relates to a vulcanizable composition comprising achloroprene rubber and an amino acid, and a cured chloroprene rubberobtained therefrom.

Heretofore, metal oxides such as magnesium oxide, zinc oxide and leadoxide have been known as vulcanized agents for chloroprene rubbers, anda combination of zinc oxide and magnesium oxide has gained mostwidespread acceptance. In commercial operations, these known vulcanizersneed to be used together with vulcanization accelerators in order toshorten the vulcanization time to an economically feasible one andimpart good properties to the resulting vulcanized rubber. Variousspecies of the vulcanizing accelerators are known, but2-mercaptoimidazoline has proved to be an especially superioraccelerator and come into almost exclusive commercial use. Recentinvestigations, however, have ascertained the tumorigenic andteratogenetic properties of the 2-mercaptoimidazoline, as reported inJournal of The National Cancer Institute, Vol. 42, page 1101 (1969) andRubber World, Januray 1975, page 41, and it has been strongly desired todevelop vulcanizers which do not require the conjoint use of such avulcanization accelerator.

The metal oxide vulcanizers remain in the final rubber products, and mayooze during use to become hazardous to the human health. Accordingly,this toxic hazard has precluded such rubber goods from applications inthe fields of medicines and foodstuffs in the form of, for example,bottles of stoppers, and has also limited the utility of the finalrubber goods. Furthermore, during the processing of rubber, the metaloxide powder scatters off and may cause adverse effects to the health ofthe working personnel. It is very desirable therefore to developnon-toxic vulcanizers that can replace the conventional metal oxidevulcanizers.

It has now been found that a certain kind of amino acid (to be referredto as a first kind of amino acid) has a superior vulcanizing action onchloroprene rubbers, and another kind of amino acid (to be referred toas a second kind of amino acid) has a superior vulcanizationaccelerating action on metal oxide vulcanizers although it does notpossess a vulcanizing action on chloroprene rubbers. This discovery ledto the solution of the above-mentioned problems.

According to this invention, there is first provided a vulcanizablecomposition comprising a chloroprene rubber and at least one amino acid(belonging to the first kind of amino acid described above) selectedfrom the group consisting of arginine, lysine, hydroxylsine, ornithine,cystine, asparagine, glutamine and citrulline.

According to another aspect of the invention, there is provided acurable composition comprising a chloroprene rubber, at least one aminoacid selected from the group consisting of arginine, lysine,hydroxylysine, ornithine, cystine, asparagine, glutamine and citrulline,and at least one metal oxide selected from the group consisting ofmagnesium oxide and zinc oxide.

The invention further provides a curable composition comprising achloroprene rubber, at least one amino acid (the second kind of aminoacid described above) selected from the group consisting of tyrosine,threonine, tryptophan, proline and hydroxyproline, and at least onemetal oxide selected from the group consisting of magnesium oxide andzinc oxide.

The present invention also provides vulcanized rubbers obtained byvulcanizing the aforesaid vulcanizable compositions.

FIGS. 1 to 4 of the accompanying drawings represent the vulcanizationcurves of the rubber compositions described in Examples of the presentinvention to be given hereinbelow.

The present invention will be described below in greater detail.

A first preferred embodiment of the present invention is a compositioncomprising a chloroprene rubber and a vulcanizing agent consisting onlyof a first kind of amino acid. It was not known prior to the presentinvention that since the first kind of amino acid defined in the presentinvention has a superior vulcanizing action on chloroprene rubbers evenwhen used alone, the use of this amino acid alone as a vulcanizing agentcan lead to the vulcanization of chloroprene rubbers. While theconventional metal oxide vulcanizers require the conjoint use ofvulcanization accelerators, it is surprising that some preferred aminoacids of the first kind specified in the present invention, for example,ornithine or lysine, afford a very high rate of vulcanization in theabsence of culcanization accelerators, and the resulting vulcanizedrubbers have good properties. The composition of this inventioncomprising a chloroprene rubber and a vulcanizer consisting only of anamino acid of the first kind does not contain a metal oxide, andtherefore has an important significance in that it increases the utilityof chloroprene rubbers and enables them to be used in the fields ofmedicines and foodstuffs.

A mixed vulcanizer consisting of a mixture of an amino acid of the firstkind and magnesium oxide and/or zinc oxide is also very good vulcanizerfor chloroprene rubbers. Hence, a composition comprising a chloroprenerubber and the mixed vulcanizer is a second preferred embodiment of thepresent invention. In fact, as will be shown in Example 4 below, themixed vulcanizer consisting of the first kind of amino acid and themetal oxide affords an increased rate of vulcanization, and givesvulcanized rubbers having good properties as compared with a vulcanizerconsisting only of the corresponding amino acid of the first kind. Sincethis mixed vulcanizer contains a metal oxide, the resulting vulcanizedrubber should not be used in the fields of medicines or foodstuffs.However, in fields which do not require any consideration of such healthregulations, this mixed vulcanizer becomes a very superior novelvulcanizer for chloroprene rubbers. Moreover, it offers an importantadvantage that unlike the conventional vulcanizer for chloroprenerubbers, the mixed vulcanizer in accordance with this invention does notrequire a vulcanization accelerator such as 2-mercaptoimidazoline.

Chloroprene rubbers used in this invention are known per se. Thechloroprene rubbers are classified into sulfur-modified type rubbers andnon sulfur modified type rubbers, but in the present invention, bothtypes of chloroprene rubbers can be used as required. Chloroprenerubbers carboxylated with an acidic monomer such as methacrylic acid canalso be used. The chloroprene rubbers used in this invention may be inthe form of solid or latex.

Vulcanization of chloroprene rubbers with the vulcanizer consisting onlyof the amino acid or both the amino acid or magnesium oxide and/or zincoxide can be carried out under the temperature (for example, 140° to180° C) and pressure conditions usually employed for chloroprenerubbers.

When the vulcanizer consisting only of the amino acid is used, itsamount (the total amount of the amino acid) is 0.01 to 50 parts byweight, preferably 0.5 to 20 parts by weight, per 100 parts by weight ofthe chloroprene rubber. When the mixed vulcanizer consisting of theamino acid and the metal oxide is used, the amout of the metal oxide is0.01 to 30 parts by weight, preferably 0.1 to 5.0 parts by weight, perpart by weight of the entire amino acid, and the amount of the mixedvulcanizer is 0.06 to 50 parts by weight, preferably 0.6 to 15 parts byweight, per 100 parts by weight of the chloroprene rubber.

A vulcanizable composition comprising an amino acid of the second kinddefined hereinabove, a chloroprene rubber and a metal oxide selectedfrom the group consisting of magnesium oxide and zinc oxide is a thirdpreferred embodiment of the present invention. Unlike the first kind ofamino acid, the second kind of amino aid does not substantially exhibita vulcanizing action on chloroprene rubbers when used alone, butexhibits a marked effect of accelerating the vulcanization ofchloroprene rubbers with the metal oxide vulcanizer. It is thereforeused as a novel vulcanization accelerator in place of the knownvulcanization accelerators such as 2-mercaptoimidazoline. The aminoacids of the second kind are quite harmless to the human health. Inspite of the recognition that 2-mercaptoimidazoline causes a toxichazard to humans, the rubber industry has stuck to it in commercialoperations because of the failure to find any suitable non-toxicsubstitute. The discovery of the aforesaid non-toxic and superiorvulcanization accelerators by the present invention, therefore, is animportant contribution to rubber technology and has marked a notableadvance in the rubber industry.

Vulcanization of chloroprene rubbers with the vulcanizer consisting ofthe second kind of amino acid and the metal oxide can be carried outunder the conventional temperature (for example, 140° to 180° C) andpressure conditions used for chloroprene rubbers.

The amount of the metal oxide is 0.01 to 30 parts by weight, preferably0.1 to 5.0 parts by weight, per part by weight of the total amount ofthe amino acid, and the amount of the mixed vulcanizer is 0.1 to 50parts by weight, preferably 1 to 15 parts by weight, per 100 parts byweight of the chloroprene rubber.

In any of the three embodiments of the vulcanizable compositions of thisinvention, other conventional compounding chemicals, such as reinforcingagent, processing aid, pigment, softening agent or plasticizer, can beincorporated as required.

Vulcanized chloroprene rubbers obtained by vulcanizing the vulcanizablecompositions of this invention can be conveniently used in variousapplications, for example, cable and wire coverings, printing rolls,window frames, antivibratory rubbers, hoses, belts, rubber-coatedcloths, sponges, shoe soles, and linings.

The following Examples further illustrate the present invention. Unlessotherwise indicated, all parts are by weight.

The various properties shown in the Examples were measured by thefollowing test methods.

Tensile strength, elongation and modulus

The measurement is made in accordance with JIS K-6301 using a Schoppertype tensile tester at a pulling speed of 500 mm/min.

Hardness

The measurement is made in accordance with JIS K-6301 using a JIS-A typehardness tester.

EXAMPLE 1

    ______________________________________                                        Chloroprene rubber *(1)                                                                              100    Parts                                           SRF carbon black *(2)  40                                                     TE-58A *(3)            1.5                                                    Arginine               4.3                                                    50% Aqueous solution of                                                       lysine                 3.2                                                    ______________________________________                                    

The chloroprene rubber *(1) was Skyprene B-10 (a registered trademarkfor a non sulfur-modified type chloroprene rubber produced by Toyo SodaKogyo Kabushiki Kaisha). The SRF carbon black *(1) was semi-reinforcingfurnace carbon black added as a reinforcing agent. The TE-58A *(3) was aproduct of Technical Processing Company, U.S.A. which was an alkalimetal salt of a higher fatty acid added as a processing aid.

The above ingredients were compounded ant the resulting compositionvulcanized, in accordance with the conventional methods shown below.

The chloroprene rubber was wound onto 8-inch open rolls with a rollinterspace of 1.4 mm. The surface temperature of the rolls wasmaintained at 50° ± 5° C. When the surface of the rubber became even, apredetermined amount of carbon black was added occasionally so that thesurface of the rolls became uniform. The addition was effected whileadjusting the roll interspace occasionally so that the bank of the rollbecame constant. After the addition, TE-58A was added, and then amixture of the lysine ad arginine.

After the addition, three three-fourths cuts were effected each way, andthe batch was cut from the roll and rolled up. The roll interspace wasnarrowed to 0.7 mm, and the rolled stock was passed endwise through theopen rolls six times. The batch was released, and aged for one day.After aging, the rolled stock was passed endwise six times through thesame open rolles as used in mixing whose surface temperature wasmaintained at 50° ± 5° C. and whose interspace was 1.4 mm. Then, thebatch was wound onto the rolls and subjected to three three-fourths cutseach way. The rubber compound obtained was sheeted, and vulcanized for30 minutes by means of a steam press at 160° C.

The properties of the resulting vulcanized rubber as shown in Table 1.

                  Table 1                                                         ______________________________________                                        Tensile strength (Kg/cm.sup.2)                                                                      116                                                     Elongation (%)        530                                                     Hardness              60                                                      300% Modulus (Kg/cm.sup.2)                                                                          60                                                      ______________________________________                                    

The experimental results shown in Table 1 demonstrate that thechloroprene rubber can be vulcanized with a mixture of lysine andarginine.

EXAMPLE 2

For this Example shows the results of experiments in which a nonsulfur-modified type chloroprene was vulcanized with other amino acidsof the first kind as a vulcanizer. Table 2 shows compounds used whichwere prepared using open rolls in the same way as in Example 1. Thevulcanizing conditions for the compound were the same as in Example 1.In Runs Nos. 1 to 3, Skyprene B-10 (registered trademark) was used asthe chloroprene rubber, and in Run No. 4, Skyprene B-30 (a registeredtrademark for a non sulfur-modified type chloroprene rubber made by ToyoSoda Kogyo Kabushiki Kaisha). The properties of the resulting vulcanizedrubbers are shown in Table 2.

A vulcanization curve of the compound prepared in Run No. 1 was drawn bymeans of an oscillating disc rheometer with the temperature adjusted to160° C., and shown in FIG. 1. FIG. 1 shows that after a suitable lengthof induction period (about 3 minutes), an abrupt rise in torque occurs.This substantiates that ornithine is very preferable as a vulcanizer.Generally, the preferred induction time in vulcanization is neithershort nor long, and the induction period of about 3 minutes is desirablefrom the viewpoint of both operation and economy. Furthermore, since anabrupt rise in torque can lead to the shortening of the heating time, itis economically very advantageous.

                  Table 2                                                         ______________________________________                                        Run No.          1       2       3     4                                      ______________________________________                                        Chloroprene rubber                                                                             100     100     100   100                                                     parts   parts   parts parts                                  SRF carbon black *(2)                                                                           40      40      40    40                                    TE-58A *(3)       3       3       4     5                                     Ornithine 50% aqueous                                                         solution          3      --      --    --                                     Lysine 50% aqueous                                                            solution         --       3      --    --                                     Glutamine        --      --       6    --                                     Asparagine       --      --      --     5                                     Tensile strength (Kg/cm.sup.2)                                                                 172     154     110   139                                    Elongation (%)   300     460     560   530                                    Hardness          37      52      50    60                                    300% Modulus (Kg/cm.sup.2)                                                                     172      89      47    61                                    ______________________________________                                         *(2) and *(3) were the same as in Example 1.                             

EXAMPLE 3

This example shows the results of experiments wherein sulfur-modifiedtype chloroprene rubber was vulcanized with an amino acid of the firstkind. Table 3 shows compounds used which were prepared in the same wayas in Example 1 and then vulcanized under the same conditions as inExample 1. The properties of the resulting vulcanized rubbers are shownin Table 3. Vulcanization curves of the compounds in Runs Nos. 2 and 3were prepared in the same way as in Example 2, and are shown in FIG. 2.

                  Table 3                                                         ______________________________________                                        Run No.          1        2        3                                          ______________________________________                                        Chloroprene rubber *(1)                                                                        100 parts                                                                              100 parts                                                                              100 parts                                  SRF carbon black *(2)                                                                          40       40       40                                         TE-58A *(3)      1.5      5        5                                          Arginine         4.3      --       --                                         Lysine 50% aqueous solution                                                                    3.2      --       --                                         Citrulline       --       5        --                                         Cystine          --       --       5                                          Tensile strength (Kg/cm.sup.2)                                                                 149      159      156                                        Elongation (%)   540      380      610                                        Hardness         56       57       48                                         300% Modulus (Kg/cm.sup.2)                                                                     88       114      54                                         ______________________________________                                         *(1) was Skyprene R-22 (registered trademark for sulfur-modified type         chloroprene rubber made by Toyo Soda Kogyo Kabushiki Kaisha). *(2) and        *(3) were the same as in Example 1.                                      

EXAMPLE 4

This Example shows the results of experiments in which chloroprenerubber was vulcanized with a mixed vulcanizer consisting of an aminoacid of the first kind and a metal oxide. Table 4 shows compounds usedwhich were prepared in the same way as in Example 1, and then vulcanizedunder the vulcanizing conditions shown in Table 4. The properties of theresulting vulcanized rubbers are shown in Table 4. Table 4 also showsthe experimental results obtained with the use of the first kind ofamino acid alone as a vulcanizer for comparative purposes. Vulcanizationcurves for Runs Nos. 1 to 6 were prepared in the same way as in Example2, and are shown in FIG. 3.

                                      Table 4                                     __________________________________________________________________________    Run No.     1    2    3    4    5    6                                        __________________________________________________________________________    Chloroprene *(1)                                                                          100  100  100  100  100  100                                                  parts                                                                              parts                                                                              parts                                                                              parts                                                                              parts                                                                              parts                                    SRF carbon black                                                              *(2)        40   40   50   50   60   60                                       TE-58A *(3) 1.5  1.5  4    4    5    5                                        MgO         --   5    --   --   --   5                                        ZnO         --   --   --   5    --   5                                        Arginine    2.5  2.5  --   --   --   --                                       Lysine 50% aqueous                                                            solution    2.5  2.5  --   --   5    5                                        Ornithine 50%                                                                 aqueous solution                                                                          --   --   4    4    --   --                                       Vulcanization                                                                 conditions                                                                    Temperature (° C)                                                                  160  160  160  160  160  160                                      Time (minutes)                                                                            30   30   20   15   15   15                                       Method      steam                                                                              steam                                                                              electro-                                                                           electro-                                                                           electro                                                                            electro-                                             press                                                                              press                                                                              thermal                                                                            thermal                                                                            thermal                                                                            thermal                                                        press                                                                              press                                                                              press                                                                              press                                    Tensile strength                                                              (Kg/cm.sup.2)                                                                             110  184  139  181  130   165                                     Elongation (%)                                                                            490  420  360  340  370  310                                      Hardness    60   68   54   60   58   72                                       300% Modulus                                                                  (Kg/cm.sup.2)                                                                             65   141  121  158  105  160                                      __________________________________________________________________________     *(1) was Skyprene B-10 (registered trademark) in Runs Nos. 1 to 4, and        Skyprene B-30 (registered trademark)                                     

The results shown in Table 4 and FIG. 3 demonstrate that the mixedvulcanizer, in comparison with the vulcanizer consisting only of thefirst kind of amino acid, affords a higher rate of vulcanization andgives vulcanized rubbers having higher tensile strength, hardness and300% modulus.

EXAMPLE 5

This Example shows the results of experiments wherein chloroprene rubberwas vulcanized with a mixed vulcanizer consisting of an amino acid ofthe second kind and a metal oxide. Table 5 shows compounds used whichwere prepared in the same way as in Exaple 1, and vulcanized under thesame conditions as in Example 1. Run No. 9 is a comparison which usedhistidine not belonging to any of the amino acids of the first andsecond kinds specified in the present invention. The properties of theresulting vulcanized rubbers are shown in Table 5. Vulcanization curvesof the compounds in Runs Nos. 10 and 11 were prepared in the same way asin Example 2, and are shown in FIG. 4.

                                      Table 5                                     __________________________________________________________________________                                          (Compa-                                                                       rison)                                  Run No.    1  2   3  4  5   6   7  8  9    10 11 12 13                        __________________________________________________________________________    Chloroprene                                                                              100                                                                              100 100                                                                              100                                                                              100 100 100                                                                              100                                                                              100  100                                                                              100                                                                              100                                                                              100                       rubber *(1)                                                                              parts                                                                            parts                                                                             parts                                                                            parts                                                                            parts                                                                             parts                                                                             parts                                                                            parts                                                                            parts                                                                              parts                                                                            parts                                                                            parts                                                                            parts                     SRF-L carbon                                                                  black *(2) 40 40  40 40 40  40  40 40 40   40 40 40 40                        TE-58A *(3)                                                                              5  5   5  5  5   5   5  5  5    5  5  5  5                         Metal oxide                                                                   ZnO        5  5   5  -- --  --  5  5  5    5  5  5  5                         MgO        -- --  -- 5  tr 5                                                                              5   5  5  5    -- -- -- --                        Amino acid                                                                    Parts      -- 5   5  -- 5   5   -- 5  5    -- 5  -- 5                         Type       -- Tyro-                                                                             Tri-                                                                             -- Threo-                                                                            Tripto-                                                                           -- Pro-                                                                             Histi-                                                                             -- Pro-                                                                             -- Proline                                 sine                                                                              pro-  nine                                                                              phan   line                                                                             dine    line                                              phan                                                        Tensile                                                                       strength   148                                                                              168 165                                                                              113                                                                              142 131 120                                                                              136                                                                              92   125                                                                              157                                                                              174                                                                              175                       (Kg/cm.sup.2)                                                                 Elongation 580                                                                              530 520                                                                              720                                                                              510 660 680                                                                              490                                                                              680  560                                                                              440                                                                              430                                                                              380                       (%)                                                                           Hardness   52 53  57 52 56  56  53 67 54   50 53 56 60                        300% Modulus                                                                             43 61  62 29 63  47  35 71 28   43 89 106                                                                              137                       __________________________________________________________________________     *(1) was Skyprene B-10 (registered trademark) in Runs Nos. 1 to 9,            Skyprene B-30 (registered trademark) in Runs Nos. 10 and 11, and Skyprene     R-22 (registered trademark) in Runs Nos. 12 and 13.                           *(2) was semi-reinforcing furnace low structure carbon black used as a        reinforcing agent.                                                            *(3) was the same as in Example 1.                                       

The results shown in Table 5 and FIG. 4 demonstrate that when the mixedvulcanizer consisting of the second kind amino acid and the metal oxideis used, the rate of vulcanization becomes high and the tensilestrength, and 300% modulus of the vulcanized rubbers are improved ascompared with the case of using only the metal oxide as a vulcanizer.The results also show that when histidine outside the scope of thepresent invention is used, the tensile strength and 300% modulus of thevulcanized rubber are deteriorated.

What we claim is:
 1. A vulcanizable composition comprising a chloroprenerubber and at least one amino acid selected from the group consisting orarginine, lysine, hydroxylysine, ornithine, cystine, asparagine,glutamine and citrulline.
 2. The composition of claim 1 wherein thetotal amount of the amino acid is 0.01 to 50 parts by weight per 100parts by weight of the chloroprene rubber.
 3. A vulcanized rubberobtained by vulcanizing the vulcanizable composition of claim 1.